This invention relates to vent exit devices for venting and condensing gases containing steam, and more particularly to a vent exit device for vent pipes which communicate a dry well with a suppression chamber and a pressure vessel with a suppression chamber in a water-moderated reactor of a nuclear plant.
In some boiling water reactors which are classified as water-moderated reactors, there is provided a primary containment vessel which completely shuts off the reactor from the atmosphere. The primary containment vessel is generally partitioned into upper and lower sections or a dry well in which a pressure vessel is encased and a suppression chamber in which ordinary water is stored. There are provided a plurality of vent pipes which extend through a floor partitioning the dry well from the suppression chamber, each of the vent pipes having its lower end immersed in the water in the suppression chamber. The vent pipes are thus capable of transferring steam, generated in the pressure vessel, to the water pool in the suppression chamber by virtue of differential pressure in the event that an accident involving leaks of steam occurs.
The steam contained in gases passing from the dry well to the water pool in the suppression chamber at the time of such accident is condensed in the water pool, thereby preventing damage to and destruction of the primary containment vessel due to a rise in pressure caused by the leaked steam.
The vent pipes also have another function. When the pressure in the pressure vessel in which a reactor core is encased is reduced at the time the nuclear reactor is shut down, it is possible to release the steam through the vent pipes by operating a relief vent valve, into the water pool where the steam can be condensed. One type of vent exit device of the prior art comprises a tubular member of the same diameter as the vent pipes, which is closed at the bottom and formed with a multitude of openings in the side wall of a portion of the device which is immersed in the water pool.
Generally, in the event that an accident involving leaks of steam from the pressure vessel occurs, the pressure in the dry well will abruptly rise. Under such condition, if the steam from the pressure vessel and the air in the vent pipes are vented to the water pool, dynamic loads as described in the following paragraphs (I) to (V) will be transiently produced and a situation resembling the occurrence of a tidal wave will be produced.
(I) When the venting of the steam is initiated, the water in the vent pipes will be ejected at high speed into the water pool by the compressed air in the vent pipes. The water jet force generated by the streams of water will be exerted as a dynamic force on the floor of the suppression chamber.
(II) The air compressed in the vent pipes at initial stages of venting of the steam containing air will expand simultaneously as the air is ejected into the water pool through the small openings in the side walls of the vent pipes. This will cause the vent pipes to vibrate. This phenomenon is referred to as a vent lateral load due to air bubble venting.
(III) If a large volume of air is released with a rush together with the leakage steam, the pressure of the air will be instantaneously applied to the floor of the suppression chamber as it is. This is referred to as a bubble formation pressure spike.
(IV) If the air pressurized as aforementioned at the time of initiation of venting of the steam containing air to the water pool through the vent pipes, the air will expand due to a sudden reduction in pressure. This will cause the water in the suppresion chamber to move upwardly and this rise in the liquid level of the water pool will cause an upwardly directed force to be exerted on the structures in the upper portion of the primary containment vessel. This is referred to as a pool swell impact.
(V) At the time of an accident, if condensation of the steam takes place continuously and the temperature of the water pool rises, or if the volume of steam vented through the vent pipes increases, the condensation capabilities of the water pool will reach a critical state. In such case, the ejected steam will not be condensed at once, and the steam will be suddenly condensed after growing into bubbles of a substantial size. This phenomenon of growing of the steam into bubbles and then being suddenly condensed is repeated, thereby causing the exits of the vent pipes to vibrate. This phenomenon is referred to as a steam condensing vibration due to steam venting.
There are possibilities that the phenomena described in the above paragraphs (I and IV) occur during the process of venting gases through the relief vent valve when a nuclear reactor is shut down. The loadings that most strongly affect the installation are those which are described in paragraphs (I), (III) and (V).
The vent exit device of the prior art mentioned above is not capable of damping the dynamic forces generated when the phenomena described in paragraphs (I) to (V) occur. This makes it necessary to design the primary containment vessel by taking these phenomena into consideration. Thus the prior art device has the disadvantage of increasing the construction expenses of a nuclear plant.
This invention has as its object the provision of a vent exit device which is capable of effectively damping the dynamic loads which are produced when the steam is vented and condensed, whereby constructional expenses of a nuclear reactor can be reduced.
The present invention is based on the principle that, when steam containing gases is led to the water pool to condense the steam by venting the gases through conduits to the water pool, a portion of each of the conduits is enlarged, so that the gases can be dispersed widely in the water pool and the energy of the gases can be rapidly absorbed by the water pool.
According to the invention, there is provided a vent exit device comprising a first elongated pipe, a second pipe larger in diameter than the first pipe and essentially immersed in a water pool, an upper cover plate connecting these two pipes together in airtight relationship, a lower cover plate adapted to close an open end of the second pipe opposite the end thereof at which the upper cover plate is applied, and a multitude of openings formed in the second pipe.